1. Field of the Invention
This invention relates to a video processing circuit for superimposing a character signal on a video signal.
2. Description of the Related Art
Recent television receivers are so devised as to indicate, for example, a channel with a number on the television screen and the sound volume by increasing or decreasing the number of vertical bars; in other words, such a television receiver displays characters such as symbols, numerals and the like on the screen. In order to prevent the screen from flickering or noise resulting in unwanted signals appearing on the picture when one channel is switched to another, a blue background (a blue-back screen) has been adopted so that the whole background of the television screen turns blue when or after channels are switched. Furthermore, operations relative to a video recording/reproducing apparatus, for example, reserving video tape recording (setting of a video tape recording date, designating of a channel, etc.), adjusting the internal clocking of the apparatus and the like are displayed on the screen as a menu.
When integrated circuits for character display are designed, taking various measures and considerations is needed to prevent the circuits for processing video signals from being affected by external noise because the video signals are easily affected thereby. On the other hand, character signals and control circuits are difficult to deal with because the character signals and the control circuits are not so much affected by external noise.
In conventional circuits for processing character signals, a high-speed switching control signal is used to switch between video and character signals in a superimposing circuit in order to superimpose the character signal on the video signal. Consequently, a high-speed switching signal is needed and this develops a problem of causing the switching signal to interfere with peripheral circuits and this results in producing interfering noise. Still another problem is what arises from a timing lag due to disturbance of the synchronization of the character signal with the switching signal.
Moreover, crystal resonators are needed for a reference frequency generator of a character signal generator, a color signal generator for generating a monochromatic (blue-back) screen and a switching signal generator for superimposing the character signal on the video signal. When a changeover switch is used for superimposing the character signal on the video signal, a switching operation in the order of one microsecond is needed and such a high-speed switch is very expensive. Consequently, switching noise is likely to affect the peripheral circuits. It is therefore extremely difficult to incorporate the switching circuit in the IC circuit for video signal processing. In addition, a special signal line is required to effect control over the high-speed switching operation; in other words, a great deal of labor and cost may be incurred to solve those many problems mentioned above.
Japanese Patent Unexamined Publication No. Sho. 63-30089, which is not technically and directly related to the present invention though, discloses a technique of employing a frequency four times higher than a color subcarrier frequency for measuring instruments. More specifically, as described in that publication, xe2x80x9cat the time of taking steps of adjusting and inspecting the characteristics during the manufacture of a color television camera, it is necessary to measure waveforms of a composite video signal as an output of the camera, of a color difference signal synchronizing with the composite video signal and the like . . . . a signal having a frequency four times higher than the color subcarrier frequency, that is, a frequency of 4 fsc is used as, for example, a timing signal for controlling various kinds of measurement.xe2x80x9d That is, it is disclosed therein to employ a signal of 4 fsc for measuring instruments during the manufacture of the camera.
Conventional video processing circuits for processing character signals will now be described with reference to the accompanying drawings. FIGS. 4 to 6 are block diagrams illustrating the conventional video processing circuits for processing character signals. As shown in FIG. 4, a composite video signal is supplied from a tuner 1 to a composite video signal processing circuit 2 and then to an OSD (On Screen Display) circuit 3. A microcontroller 4 generates a control signal and supplies the control signal to the OSD circuit 3 via a control bus 5. In other words, the microcontroller 4 sends, to the OSD circuit 3 via the control bus 5, control signals in the form of code signals representing various characters, for example, a numeral for indicating a channel number, several vertical bars for indicating the sound volume, characters for indicating reproducing and recording operations when a VCR is operated and the like by the use of a means for detecting any one of the operations performed by a viewer or an operating condition that has been inputted. Further, the microcontroller 4 generates and supplies a control signal to the OSD circuit 3 so that these characters are added to the video signal in the OSD circuit 3 or a picture is generated therein as a substitute. A clock oscillator with a crystal resonator 6 (resonance frequency: 16 MHz) externally attached thereto is set in the microcontroller 4, and the OSD circuit 3 is controlled by the control signal outputted from the microcontroller 4. The video signal is supplied to the OSD circuit 3 in which characters are added to the video signal, which is then supplied to a video signal output circuit (not shown) at the following stage. In a case where the output video signal of the video signal processing circuit 2 is so bad in quality that it is not fit for viewing and listening or where a menu and the like are desired to be displayed on the whole screen, the blue background (blue-back) generated by the OSD circuit 3 is made to appear on the screen under the control by the microcontroller 4.
In this circuit arrangement, a crystal resonator 7 for generating a color subcarrier signal (3.58 MHz) is provided in the color synchronizing circuit incorporated in the video signal processing circuit 2 and a crystal resonator (14.3 MHz) 8 is provided in the OSD circuit 3 and besides the crystal resonator (16 MHz) 6 is also provided in the microcontroller 4 as described above. However, three oscillating circuits by means of the three crystal resonators are considerably costly to maintain their stable operation against temperature variation, variation with time and the like.
FIG. 5 shows a circuit further developed from which is shown in FIG. 4, wherein an OSD circuit is incorporated in the microcontroller 4 and the circuit arrangement is much simplified and operated in the same way as what is shown in FIG. 4. Even in this case, three crystal resonators 6, 7 and 8 are required likewise as in the case of FIG. 4.
The conventional circuits shown in FIGS. 4 and 5 poses the following problems.
(a) Since the microcontroller 4 always has the built-in clock oscillator (usually with a frequency of 10+several MHz and an amplitude of 5 V), the video signal is affected by the output of the clock oscillator.
(b) Conversely, a microcomputer circuit may be affected by the video signal when a video signal having a high energy level is passed therethrough.
(c) The conventional microcontroller 4 has the built-in driving circuit (not shown) of a fluorescent display tube (FLD) and when the video signal is passed through the microcontroller 4, the video signal is affected by the noise generated by the driving circuit of the fluorescent display tube.
(d) Even in a circuit having no built-in driving circuit of the fluorescent display tube, the video signal passing through the microcontroller 4 is affected by the noise generated by the driving circuit of the fluorescent display tube disposed close to the microcontroller 4.
Thus, the conventional circuits have many problems as described above and the disadvantage is that a great deal of labor and cost may be incurred when some measures are taken to solve these problems.
FIG. 6 shows still another conventional circuit. The microcontroller 4 has functions of generating characters, a blue-back screen and the like out of those performed by the OSD circuit. Moreover, an external switching circuit 9 performs the operation of adding characters to the video signal and making the generated blue-back screen appear alternatively. This operation is substantially the same as what is shown in FIG. 4. However, the influence of noise deriving from the driving circuit of the fluorescent display tube is reduced because the video signal in the video signal processing circuit 2 is separated from the driving circuit of the fluorescent display tube.
Even in this case, however, the following problems develop and the problem of necessitating many crystal resonators has not been solved yet.
(i) At the time of the aforementioned superimposition, the switching circuit 9 has to perform the switching operation in the order of one microsecond and therefore a high-speed, expensive switching circuit is needed.
(ii) Since the switching circuit 9 performs the high-speed switching operation, the peripheral circuits may be affected by the switching noise.
(iii) Consequently, it is difficult to incorporate the switching circuit in the video IC of the video signal processing circuit.
(iv) A special signal line becomes necessitated because control is specifically needed to effect the high-speed switching.
Even this circuit has many problems to be solved and a great deal of time and cost may be incurred when some measures are taken to solve these problems. Although there is a document (e.g., Toshiba Review 1994, Vol. 49, No. 7, Item 2.5 on page 516) describing the provision of a built-in switching circuit in the video signal processing circuit 2, simply providing the built-in switching circuit still poses the same problems as mentioned above and has not been led to complete solution.
It is an object of the present invention to provide a video processing circuit fit for integration by solving the problems pertaining to the conventional video processing circuit for processing a character signal, the problems including necessitating a high-speed switching signal in a superimposing circuit when the character signal is superimposed on a video signal because the video signal and the character signal are switched with the high-speed switching control signal, causing peripheral circuits to be badly affected by switching noise accompanied by a high-speed switching operation (noise interference when an analog circuit and a digital circuit are integrated into one IC), causing a timing lag by disturbance of the synchronization of the character signal with the switching signal, and difficulties in the circuit integration.
Furthermore, it is another object of the present invention to provide a video processing circuit fit for integration and capable of solving the problems pertaining to the conventional video processing circuit, the problems including necessitating a plurality of crystal resonators and an expensive high-speed switch, difficulties in making an IC circuit for processing a video signal incorporate the switching circuit, and a great deal of cost incurred by taking some measures to solve the problems.
In order to achieve the above objects, the present invention provides a video processing circuit for receiving a video signal from a tuner or a video recording/reproducing apparatus through a first or second input terminal and for outputting the video signal as it is through an output terminal under one condition and outputting a character signal from a signal source thereof instead of the video signal through the output terminal or outputting the video signal with the character signal superimposed thereon from the output terminal under the other condition, wherein among a plurality of circuits interposing from the first and second input terminals to the output terminal, only circuits related to the signal source of the character signal are formed in one integrated circuit and the other circuits are formed in the other integrated circuit.